Wave guide impedance transformer



R. H. DICKE March 9, 1954 WAVE GUIDE IMPEDANCE TRANSFORMER Original Filed March 8, 1945 INVENTOR.

ROBERT H. DICKE Y ATTORNEY UNITED STATS E ATENT OFFICE WAVE GUIDE IMPEDANCE TRANSFORMER Robert H. Dicke, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Original application March 8, 1945, Serial No. 581,695, now Patent No. 2,593,120, dated April Divided and this application March 19, 1952, Serial No. 282,036

2 Claims. 1

This application is a division of application Serial No. 581,695, entitled Transmission Systems, which was filed March 8, 1945, and issued April 15, 1952, as Patent No. 2,593,120.

This invention relates to transmission systems and more particularly to an impedance transformer for use with ultra high frequency energy.

According to conventional theory the dominant mode of operation of a wave guide may be defined as that condition of operation in which the conto above render it advisable to eliminate the unfiguration of electric and magnetic lines of force wanted mismatch and reflections. permits the transmission of energy at the lowest Many methods have been employed in the past possible frequency through a wave guide of a for matching or transforming one impedance to given size and geometric cross-section. It may another. At the lower radio frequencies this is also be defined with equal accuracy as that con- 1.3v done with lumped circuits such as transformers dition of operation in which the configuration of or with line stubs such as the double stub tuner. electric and magnetic lines of force permits the For radio frequencies sufilciently high to warrant transmission of energy through the smallest the use of wave guides these stub tuners can still dimensional wave guide of a given cross-section be used but with increasing difficulty. Impedat a given frequency. ance transformers with greater range of match- In a rectangular wave guide operated in the ing and ease of operation are desirable. dominant mode, there exists a sinusoidal distri- One object of this invention is, therefore, to bution of electric lines of force along the major terminate each wave guide at its junction with axis of the rectangular cross-section. These other wave guides in its characteristic impedelectric lines of force or electric field vectors are 5 ance. perpendicular to the major axis of the rectangle. Still another object of this invention is to When a main rectangular wave guide designed match or transform one impedance into another. to be operated in the dominant mode is joined In accordance with the present invention there symmetrically by a wave guide whose axis is paris provided a matched junction formed by three allel to the electric field vectors in the main wave wave guides. One of the three wave guides is guide, the juncture is said to be a series junction. joined to a second of the three wave guides in a When the joining wave guide is symmetrical to series junction. The third wave guide joins the a main wave guide and the axis of the joining second wave guide in a parallel junction. An iris, guide is perpendicular to the electric field vectors for matching purposes, is inserted in the second within the main guide, the junction is said to be wave guide. It is mounted in such a position as a parallel junction. When these two junctures, to lie in a plane which also contains the axes one series and one parallel, are made in such a of the first and third wave guides. A second iris manner that the axes of the series and parallel is located within the first wave guide. The two wave guides join the axis of the main wave guide irises are so adapted that each wave guide sees at the same point certain unique properties exist. as its termination at the junction its character- Broadly, any geometric arrangement of four wave istic impedance. guide branches will also give these unique proper- For a better and fuller understanding of the ties if the following conditions are met. The invention, together with other objects thereof, axes of the four guides must meet at a common reference is made to the following detailed depoint. In a first and second of these guide scription taken in connection with the accombranches the electric lines of force are perpenpanying drawings in which: dicular to each other and the lines of force in one Fig. 1 shows 2. improved wave guide junction; of these first and second wave guide branches Figs. 2, 3, and 4 show views looking into differmust be perpendicular to a plane passing through ent wave guides making up the junction of Fig. 1 the axes of the first and second wave guide to show the position of matching irises; branches. The third and fourth of these wave Figs. 5A, 5B, 5C, and 5D facilitate describing guide branches must be symmetrical with respect some of the unique properties of the matched to the plane passing through the axes of the first junction shown in Fig. 1; and and second wave guide branches which was just Fig. 6 shows an improved impedance transreferred to. These unique properties are exformer.

Referring now more particularly to Fig. 1, there plained further in the detailed discussion of the drawings. It will be appreciated, however, by those skilled in the art that where any abrupt change in structure occurs, mismatch and consequent undesirable reflections also tend to occur. This is especially so where one wave guide branches into two or more wave guides. When a wave guide is terminated in its characteristic impedance, no mismatch or reflections will occur. The desirability of the unique properties referred is shown a matched junction formed by three wave guides. A first wave guide with two branches II and I2 is joined by a second wave guide I3, symmetrically in a series connection. A third wave guide M is joined symmetrically and in a parallel connection to the first wave guide ll-l2. The series and parallel junctions are so made that the axes of the first, second, and third wave guides meet in a common point. Fig. 1 shows the wave guides 13 and M as being perpendicular to the wave guide H-l2. While this is a preferred embodiment, it is not desired to limit the invention here described to this geometric arrangement. Two irises l and I6, preferably in the form of thin plates of conducting material, are inserted at the junction to permit matching of all four wave guide branches, that is to say, looking from any one of the four wave guide branches toward the junction each of the. wave guide branches will be terminated by its characteristic impedance. Fig. 2 shows the iris I5 partially closing off wave guide branch H as it is viewed from line 2-2 in Fig. 1. In Fig. 3 this same iris I5 is shown as it is viewed from line 3-3 in Fig. 1. Fig. 4 again shows the iris [5. as it is viewed from line l4 in Fig. 1. Figs. 2, 3, and 4 taken togethershow the iris 15 to be within the wave guide I I-I 2 and to lie in a plane which includes the axes of wave guides l3 and M; By variation of the length i! and the depth iii of the iris [5 shown inFig. 2, the iris I5 is adjusted empirically so that the third branch 14 is terminated in its characteristic impedance. Fig. 4 also shows the position of the iris l5. Figs. 2 and 4 taken together show the iris t5 to'be within the wave guide l3. By variation of the length is of the iris 16 (Fig. 4) and the axial distance 26 of the iris 16 from the junction (Fig. 3) the iris I6 is adjusted empirically so that the second branch I3 is terminated in its characteristic impedance.

Referring now to Figs. 5A, 5B, 5C, and 5D specifically; there is shown four novel ways inwhich thematched junction of Fig. 1' operates. Al-

through there are other novel modes in which a connected wave guide H3, divides at the junction into the two branches H and i2 of main wave guide. The energy divides equally, and the two parts are in phase with each other as indicated by the designation +(P/2) at the ends of each of wave guide branches H and i2. In Fig. 5B the converse of the condition illustrated in Fig. 5A is shown. If energies of equal magnitude and similar phase +(P/2) enter the two branches H and 12 of the main wave guide, these energies combine at the junction, and all of this energy enters the parallel connected wave guide l4 and is designated by P. In Fig. 5C is shown the condition where energy P enters theseries connected wave guide It. This energy divides equally and passes into the two branches II and 12 of the main guide. The two energies will be in. 180 phase opposition as shown by the designation (P/2) at the end of guide branch II and the designation +(P/2) at the end of guide branch I2 in this figure. Fig. 5D shows the converse of the condition shown in Fig. 5C. If energies of equal magnitude and in 180 phase opposition as indicated by -(P/2) and +(P/2), the energies will combine and will all enter the series connected wave guide l3. This energy is designated by P. To those skilled in the art, it will be obvious that energies which enter the two branches of the main guide but which are neither in phase nor in phase opposition will be combined and will divide going'into both the series and the parallel connected wave guides. The relative division into the series and parallel connected wave guides will be a function of the phase relation existing between the two energies. This willbe readilyseen when one considers that either energy may be resolved into two components, onecomponent in phase and one component in phase'opposition to the other energy.

Fig. 6. shows a device employing a matched junction in a wave guide circuit as an impedance transiormer. Because of the desirability of the matched. junction, it is preferred for use in this circuit. However, the circuit will function with junctions like the one shown in Fig. 1 if the matching irises are omitted. The impedance II to be" transformedis connected to a series connected wave. guide 12 of. a junction 11. The impedance- T3 to which impedance H is to be transformed is seen when looking into a parallel connected wave guide 174-. Two plungers i5 and I8 are inserted in. wave guides 81 and 82 and are adapted to be adjusted in their relative positions. These two: plungers are located at distances D1 and D2, respectively; from the junction as shown in Fig. 7.

The method for determining the distances D1 and D2 is explained in the following part of the specification.

In a wave guide such as wave guid 12 which is terminated by an impedance H which is equal to Z1, other than its characteristic impedance Zc there exists standing: waves and associated therewith a. standing wave ratio SWR. The SWR is defined. as the: ratio of the maximum amplitude of signal along the wave guide to the minimum. amplitude of signal along the wave guides. The maximum and minimum amplitudes normally exist one fourth wavelength apart. If the impedance ft or Z1 is divided by Zc, the resulting quotient which. is acomplex notation locates a point on animpedance circle diagram known as the Smith. chart. A circle passing through this point with 1+9'O' as the center will describe the loci of all impedances existing along wave guide 12. Since we normally desire a matched system, we wish to transform Z1 to Zc. By variation of D1 and D2 it possible to match an impedance Z1 to the waveguide 12- i. e. totransform it to 20. D1 and D2, the distances from the center of the T-junction to the plungers l5 and it, can be thought of as being determined by two parameters X and Y which are functions of the two impedances Z1 and Z6 considered. The following relation holds:

(1) D1=X+Y (2) D2=XY It is normally possible to calculate or determine experimentally the SWR. If the terminating impedance Z1 and the characteristic impedance Zc are known and both are pure resistances,

then the reflection coefficient Ais given by The following expression derived from a consideration of the junction described above gives a relationship between the reflection coeificiem; A and D1 and D2.

However, since from Equations 1 and 2 (6) D1D2=2Y Equation 5 may be solved for Y. From any arbitrary value of X, Y may be set by a system of levers arranged to move the two plungers l5 and 16 equally in opposite directions from the center of the junction 71 or Y may be set for each plunger separately. This will set up the proper SWR in the wave guide containing the impedance Z1. If now we vary X, we can effectively move the standing waves until the actual impedance Z1 agrees with the value of the standing wave existing at the termination. Varying the parameter X can be thought of as rotating the Smith diagram or as effectively lengthening the wave guide between the point of matching and the terminating impedance Z1. A system of levers may be set up which will move the two plungers l5 and is in the same direction from the center of the junction and hence automatically set both plungers to conform to X.

It can be shown that any electrically symmetrical impedance transformer which will match any impedance to a wave guide will also match any impedance to any other impedance. As shown in the case above, if both plungers l5 and 16 are changed in position by the parameter Y, the SWR only is changed and the movement on the Smith chart is radial. If, however, we move only one plunger, the movement on the Smith chart will be circular. If we move the other plunger, we move along another circular path. It is possible, therefore, by altering the two plungers individually to transform any impedance to any other.

While there has been described What is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made thereon Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An electrical apparatus for transforming a first impedance to a second impedance, said apparatus comprising pipe wave guide means having first, second, third, and fourth branches, said first, second, third, and fourth branches being so joined that their axes meet in a common junction point, said first and second branches having a common axis and extending respectively in opposite directions from said common junction point, whereby said first and second branches form a main pipe wave guide, said third branch being series connected to said main wave guide, and said fourth branch being parallel connected to said main wave guide; a first iris inserted in said main wave guide in a plane containing axes of said third and fourth branches; a second iris inserted in said third branch, said first and second irises being adapted to cause each of said first, second, third and fourth branches to be terminated at the common junction point in its own characteristic impedance; a first movable shorting member inserted in and terminating the open end of said first branch; a second shorting member inserted in and terminating the open end of said second branch; said first impedance terminating the open end of one of said third or fourth branches; and means coupled to said first and second movable shorting members for positioning said first and second movable shorting members to predetermined distances from the center of said junction for obtaining said transformed second impedance at the open end of the other of said third or fourth branches.

2. An electrical apparatus for transforming a first impedance to a second impedance, said apparatus comprising pipe wave guide means having first, second, third, and fourth branches, said first, second, third, and fourth branches being so joined that their axes meet in a common junction point, said first and second branches having a common axis and extending respectively in opposite directions from said common junction point, whereby said first and second branches form a main pipe wave guide, said third branch being series connected to said main wave guide, and said fourth branch being parallel connected to said main wave guide; a first movable shorting member inserted in and terminating th open end of said first branch; a second shorting member inserted in and terminating the open end of said second branch; said first impedance terminating the open end of one of said third or fourth branches; and means coupled to said first and second movable shorting members for positionin said first and second movable shorting members to predetermined distances from the center of said junction for obtaining said transformed second impedance at the open end of the other of said third or fourth branches.

ROBERT H. DICKE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,410,840 Samuel Nov. 12, 1946 2,445,895 Iyrrell July 27, 1948 

